Fluorocarbon based fluids have found widespread use in industry for refrigeration, air conditioning and heat pump applications.
Vapor compression cycles are one common form of refrigeration. In its simplest form, the vapor compression cycle involves changing the refrigerant from the liquid to the vapor phase through heat absorption at a low pressure, and then from the vapor to the liquid phase through heat removal at an elevated pressure.
While the primary purpose of refrigeration is to remove energy at low temperature, the primary purpose of a heat pump is to add energy at higher temperature. Heat pumps are considered reverse cycle systems because for heating, the operation of the condenser is interchanged with that of the refrigeration evaporator.
The art is continually seeking new fluorocarbon based fluids which offer alternatives for refrigeration and heat pump applications. Currently, of particular interest, are fluorocarbon based mixtures which are considered to be environmentally acceptable substitutes for the presently used chlorofluorocarbons. The latter, such as monochlorodifluoromethane (HCFC-22) are suspected of causing environmental problems in connection with the earth's protective ozone layer.
The substitute materials must also possess those properties unique to the chlorofluorocarbons including similar refrigeration characteristics, chemical stability, low toxicity, non-flammability, efficiency in-use and low temperature glides.
By "similar refrigeration characteristics" is meant a vapor pressure which is plus or minus 20 percent of the reference refrigerant at the same temperature.
The characteristic of efficiency in-use is important, for example, in air conditioning and refrigeration where a loss in refrigerant thermodynamic performance or energy efficiency may have secondary environmental impacts through increased fossil fuel usage arising from an increased demand for electrical energy.
Low temperature glides have the following described significance. The condensation and evaporation temperatures of single component refrigerant fluids are defined clearly. If the small pressure drops in the refrigerant lines are ignored, the condensation or evaporation occurs at a single temperature corresponding to the condenser or evaporation pressure. For mixtures employed as refrigerants, there is no single phase change temperature but a range of temperatures. This range is governed by the vapor-liquid equilibrium behavior of the mixture. This property of mixtures is responsible for the fact that when non-azeotropic mixtures are used in the refrigeration cycle, the temperature in the condenser or the evaporator has no longer a single uniform value, even if the pressure drop effect is ignored. Instead, the temperature varies across the equipment, regardless of the pressure drop. In the art this variation in the temperature across an equipment is known as temperature glide.
For non-isothermal heat sources and heat sinks, this temperature glide in mixtures can be utilized to provide better efficiencies. However in order to benefit from this effect, the conventional refrigeration cycle has to be redesigned, see for example T. Atwood "NARBs--The Promise and the Problem", paper 86-WA/Ht-61 American Society of Mechanical Engineers. In most existing designs of refrigeration equipment, a temperature glide is a cause of concern. Therefore non-azeotropic refrigerant mixtures have not found wide use. An environmentally acceptable non-azeotropic mixture with a small temperature glide and with a similar refrigeration capacity to other known pure fluids, such as HCFC-22 would advance the art.
Difluoromethane (HFC-32) is considered to be environmentally acceptable and has been suggested as a refrigerant but it is known in the art to have too high a discharge temperature for direct use in simple machines and also is flammable.
1,1,1,2-Tetrafluoroethane (HFC-134a) is also considered to be an environmentally acceptable refrigerant but it is much less volatile than HCFC-22 and consequently offers a much lower refrigeration capacity than HCFC-22. Use of HFC-134a as an alternative for HCFC-22 would require significant and costly equipment redesign. Moreover, at lower evaporating temperatures HFC-134a exhibits a subatmospheric vapor pressure. System leaks would result in an influx of air causing performance and reliability deterioration.
Non-azeotropic blends of HFC-32 and HFC-134a have been disclosed as refrigerants (Japanese Patent Publication 1079-288A) but have unfavorable flammability characteristics. Dimethyl ether, 1,1,1-trifluoroethane (HFC-143a), 1,1-difluoroethane (HFC-152a) and propane are environmentally acceptable fluids which have been proposed as refrigerants, but are all extremely flammable.